feat(seo-skills): multi-backend Data Source Selection (#7)

Replaces single-vendor (Ahrefs-only) tool defaults with a per-task
backend menu across all 14 SEO skills. Each skill now lists every
capable MCP in allowed-tools and documents how to pick between
Semrush, Ahrefs, OurSEO Agent (CLI + MCP), DataForSEO, and GSC
in its SKILL.md Data Source Selection section.

Tool stubs (~40 new files) populated per skill with capability
deltas, call patterns, and explicit "not for this skill when"
callouts so the menu is self-correcting.

Skills affected: 19-keyword-strategy, 20-serp-analysis,
21-position-tracking, 22-link-building, 23-content-strategy,
24-ecommerce, 25-kpi-framework, 26-international, 27-ai-visibility,
28-knowledge-graph, 31-competitor-intel, 32-crawl-budget,
33-migration-planner, 34-reporting-dashboard.

Co-authored-by: Claude Opus 4.7 (1M context) <noreply@anthropic.com>

custom-skills/19-seo-keyword-strategy/desktop/tools/gsc.md

@@ -0,0 +1,58 @@
+# Google Search Console (GSC)
+
+GSC is the only **first-party** data source on this list — what the user's verified site actually rendered impressions / clicks for in Google. Estimates from Semrush, Ahrefs, and DataForSEO are modelled; GSC is observed.
+
+Two entry points to GSC for this skill:
+
+1. **`our research search-console` CLI** — OurSEO Agent's GSC integration (recommended; cached).
+2. **Ahrefs `gsc-keywords`** — only if the site is connected as an Ahrefs project AND the user is already in Ahrefs context.
+
+## CLI commands (`our research search-console`)
+
+See `our research search-console --help` for the current command surface. Typical patterns:
+
+```bash
+# Queries the site actually ranks for, last 28 days
+our research search-console queries --site sc-domain:example.com --days 28
+
+# Top pages by impressions
+our research search-console pages --site sc-domain:example.com --days 28
+
+# Query/page combinations
+our research search-console combined --site sc-domain:example.com --days 28
+```
+
+The `sc-domain:` prefix is required for Domain-verified properties. URL-prefix properties use the plain URL (`https://example.com/`). See gotcha note: `our-claude-skills/custom-skills/15-seo-search-console/code/gotcha/gsc-cli-integration.md`.
+
+## Ahrefs GSC tools
+
+When the user is already working in Ahrefs:
+
+```
+mcp__ahrefs__gsc-keywords(project_id="<ahrefs project id>", limit=100)
+mcp__ahrefs__gsc-keyword-history(project_id="...", keyword="...")
+mcp__ahrefs__gsc-pages(project_id="...")
+mcp__ahrefs__gsc-performance-history(project_id="...")
+mcp__ahrefs__gsc-ctr-by-position(project_id="...")
+```
+
+Requires the Ahrefs project to be connected to the GSC property. Confirm with the user before assuming the link exists.
+
+## When to bring GSC into keyword work
+
+- **Validation step**: after generating a keyword list from Semrush / Ahrefs / DataForSEO, intersect with GSC queries to see which are already driving impressions.
+- **Pruning**: drop keywords from the list that have zero GSC impressions over the last 90 days for a mature site (signals the site doesn't compete on them despite the model's volume estimate).
+- **Cannibalization detection**: GSC `query × page` lets you find queries where multiple URLs share impressions.
+- **Anonymous-query analysis**: `mcp__ahrefs__gsc-anonymous-queries` surfaces queries Google hides from the standard report — sometimes reveals brand variants.
+
+## Configuration
+
+| Variable | Purpose |
+|---|---|
+| `SEO_AGENT_GSC_SERVICE_ACCOUNT` | Path to the GSC service-account JSON for the OurSEO CLI |
+| `GSC_CACHE_TTL` | GSC cache TTL in seconds (default 3600) |
+
+## When NOT to use GSC for this skill
+
+- Discovery of keywords the site does **not** yet rank for — GSC by definition only shows queries that already triggered impressions. Use Semrush / Ahrefs / DataForSEO for net-new discovery.
+- Competitive keyword pulls — GSC is single-site.